Raw natural gas and other hydrocarbon streams often contain naturally occurring contaminants, such as, for example, water vapor, CO2, H2S, and mercaptans, and other sulfur compounds. In the case of raw natural gas, other contaminants, such as methanol or glycol, are sometimes purposely added at the natural gas field production facility to prevent the formation of hydrates or ice in the natural gas production stream while it is in transit to the treating facility. Whether such contaminants occur naturally in the gas or are purposely added, they must be substantially removed prior to use of the natural gas in certain industrial or residential applications.
One method of removing contaminants from hydrocarbon streams, including raw natural gas streams, involves the use of solid adsorbents which include, for example, alumina, silica gel, activated carbon and molecular sieves such as zeolites. These materials are typically used in packed beds. Typically, a contaminated hydrocarbon stream, either in gaseous or liquid form, is passed through the bed and the adsorbent materials in the beds preferentially adsorb the contaminants, thereby reducing their concentration in the hydrocarbon stream effluent emerging from the bed.
The adsorbents eventually become saturated with adsorbed contaminants, at which point the adsorbent will no longer effectively remove the contaminants from the hydrocarbon stream. When saturation occurs, the adsorbent materials must be either replaced or regenerated. One way of regenerating an adsorbent is to pass a heated regeneration fluid stream, either in a gaseous or a liquid state, through the adsorbent bed, often in a countercurrent manner. In this way, the adsorbed contaminants are desorbed from the adsorbent and moved into the regeneration fluid stream in which they are carried out of the bed. The regeneration fluid stream can then be purified and recycled, or it can be used as fuel gas.
During regeneration, temperatures in the beds can often reach approximately 600° F. At these temperatures, particularly in the case of zeolitic molecular sieves, some adsorbed species may “crack” to form highly carbonaceous compounds, or “coke.” For example, while water and some other compounds are simply desorbed from the molecular sieves during regeneration, alcohols, glycols, heavy hydrocarbons such as benzene, toluene, and xylenes (BTEX), mercaptans, and organic sulfides and disulfides may be subject to cracking on molecular sieves during regeneration. In the case of alcohols, one possible decomposition mechanism results in the formation of the corresponding olefins (R′) and water according to equation (1):ROH+heat→R′+H2O  (1)
Under the acidic conditions of a typical mole sieve, the olefins may form polymers, and these polymers may then dehydrogenate to form coke. A similar decomposition mechanism may apply to the decomposition of mercaptans. The coke formed from such decomposition reactions plugs the pores of the molecular sieves and builds up over repeated thermal regeneration cycles, thereby hindering fluid flow through the molecular sieve bed, eventually rendering it ineffective. When this occurs, the molecular sieves can no longer be regenerated and must be replaced at potentially significant expense, including possibly expenses incurred from unplanned downtime for the hydrocarbon purification facility. Adsorbed species that may cause coking on molecular sieves include, but are not limited to, mercaptans, organic sulfides, disulfides, heavy hydrocarbons, methanol, and glycol.
In addition to deactivation by coking, molecular sieves can also undergo thermal deactivation. For this reason, it would be desirable to use a low regeneration temperature. If the regeneration temperature is too low, however, the quality of the product obtained from the regenerated molecular sieves may be inadequate. That is, if the regeneration temperature is too low to sufficiently desorb contaminants on the molecular sieves, the absorptive capacity of the regenerated molecular sieves will be low and the natural gas product or other fluid product obtained by treatment with such molecular sieves will have a contaminant level that is unacceptably high.
Thermal decomposition and decomposition of methanol, mercaptans, and other adsorbed species during thermal regeneration of molecular sieve adsorbents (coking) are well-recognized problems in the industry. Accordingly, embodiments of the present invention provide an improved method of removing contaminants from hydrocarbon streams using molecular sieves and an improved method of regenerating the molecular sieves while reducing the decomposition of adsorbed contaminants. Some embodiments of the improved methods described herein permit the use of lower overall regeneration temperatures to help reduce coking and thermal deactivation while at the same time maintaining product quality. Other embodiments incorporate such improved methods in a method for producing liquefied natural gas (LNG).